222Rn and its relation with meteorological conditions and gaseous pollutants in the outdoor environment of Qena City South of Egypt

In the outdoor environment of Qena city, located in southern Egypt, 222Rn concentrations were continuously measured from January 2015 to December 2015 using Alpha-Guard radon monitor, PQ 2000 PRO. Meteorological parameters (air temperature AT, relative humidity RH, and evaporation rate EV) and gaseous pollutants [ozone O3, nitrogen oxide NOx, and particulate matter that has a diameter of less than 10 µm (PM10)] data for the same period were collected from the meteorological station. This work aims to study the potential relationships between 222Rn and each of the meteorological parameters and gaseous pollutants. According to observational data, the yearly average concentrations of 222Rn were 19.35 ± 1.58 Bq/m3, 31.68 ± 1.97 µg/m3 for O3, 9.97 ± 2.27 µg/m3 for NOx, and 109.95 ± 6.48 µg/m3 for PM10. The concentration of 222Rn is positively correlated with relative humidity, nitrogen oxide, and particulate matter, whereas it is negatively correlated with air temperature, evaporation rate, and ozone. Descriptive statistical analysis revealed various distribution patterns for 222Rn, meteorological parameters, and gaseous pollutants.


Data collection and measurement
Alpha-Guard Radon monitor, PQ 2000PRO, was used to measure 222 Rn concentrations from January 2015 to December 2015.The monitor is designed for long-term monitoring of radon gas concentration with a 0.56 L active volume cylindrical ionization chamber operated at + 750 V.It has a sensitivity of 4.5 CPM/100 Bq m −3 and can measure in the range of 2-20,00,000 Bq m −312 .The detector was placed in the open air and continuous measurements of 222 Rn concentration in diffusion mode were performed at 60-min intervals.The average concentration of 222 Rn was calculated for three period of the day to avoid the missing measured during the day.The calibration of the Alpha-Guard took place at the calibration unit of the SAPHYM Co laboratory in Frankfurt, Germany.During the calibration, a standard radon gas generator (SRM 4968), which had an activity of 400 Bq 226 Ra and a 90% 222 Rn emanation fraction, was used to expose the Alpha-Guard.The exposure was conducted inside a gas-tight calibration chamber made of stainless steel 12 .
Meteorological parameters such as AT, RH, EV, NO x , O 3 , and PM 10 have been monitored by Standard synoptic weather station, WMO unique code 62403.It is a land surface station located in inside the campus of the South Valley University, Upper Egypt, that exhibits a dry desert climate.The site is surrounded by sandy surfaces with sparse rocks of varying sizes.
The meteorological station used Automated weather station to measure AT and RH, and used The Standard Evaporative Vessel class A for determining EV.The ambient PM10 data has been measured by using Thermo Scientific Model 5030 SHARP (Synchronized Hybrid Ambient, Real-time Particulate Monitor), while O3 has been measured by Ozone anlayser C49.

Descriptive statistics
Data analysis of 222 Rn concentrations, metrological parameters, and gaseous pollutants concentrations were conducted using Excel software.The obtained results are presented in Table 1.Each of the meteorological parameters and gaseous pollutants examined, except for AT, has positive skewness values, indicating an asymmetric distribution to higher values.Conversely, a negative skewness value indicates a distribution is skewed towards lower values 13 .The skews value of 222 Rn is zero, suggesting a uniform distribution around its mean value.
Kurtosis is a measure of the distribution's shape and whether it has a sharp or flat peak in comparison to a normal distribution.Both 222 Rn and NO x exhibit positive kurtosis, indicating that their distributions are leptokurtic.This means that their distributions have a sharp peak near the mean with a longer and wider tail, indicating wider variations in their values, with many far from the mean.Conversely, AT, RH, EV, O 3 , and PM 10 exhibit negative kurtosis, indicating that their distributions are platykurtic.This means that their distributions have a flat peak near the mean with a shorter and thinner tail, indicating lower variations in their values, with most values being around the mean 14 .

Diurnal, monthly, and seasonal variations of outdoor 222 Rn concentrations
The diurnal outdoor 222 Rn concentrations showed wide fluctuations, with maximum values recorded in the early morning and minimum values in the afternoon (Fig. 2).Highest concentration of 222 Rn 34.4 Bq/m 3 was recorded in the mornings of January, while the lowest concentration 4.1 Bq/m 3 was recorded in the afternoons of May.These variations can be attributed to the changes in the stability of the atmosphere during the day and night due to fluctuations in the temperature of the ground and surface air.The Earth's surface temperature increases during the day due to solar radiation, causing the temperature of the atmosphere near the surface to rise.This results in surface air circulation due to thermal convection motions, which, in turn, leads to radon dispersion vertically in the upper air layers.After sunset, the surface air cools, and thermal convection motions decrease, resulting in the accumulation of radon in the lower air layers.Since the highest temperatures are in the afternoon and the lowest in the early morning, radon levels decline in the afternoon and increase in the early morning 15 .The average monthly concentration of 222 Rn were variation from 8.10 Bq/m 3 in June to 30.47 Bq/m 3 in January (Fig. 3).An ANOVA (Analysis of Variance) test was conducted to investigate the monthly and seasonal variation of 222 Rn concentration.The p-value from both tests were found to be smaller than the significance level (α) of 0.05, leading to the rejection of the null hypothesis.Therefore, there is a significant difference in Rn concentration across both the months and seasons.Average seasonal variations showed that outdoor 222 Rn had the highest concentration in winter and the lowest in spring as shown in Fig. 4. The decrease in radon levels during spring may be attributed to the spring monsoons, which cause strong vertical mixing of air.The spring months are characterized by hot spring winds throughout Egypt, known to the Egyptians as the Khamasin wind, which can contribute to lower radon levels during this season 11 .This agrees with Abdel Galeil 10 finding of wind speed in Qena, where he found the highest wind speed in spring months with average value of 2.8 m/s.

Variations of metrological parameters with outdoor 222 Rn
Figure 5 shows the average monthly variations of outdoor 222 Rn concentrations and meteorological parameters such as AT, RH, EV.The values of AT, RH, and EV ranged from 15.00 °C, 19.45%, 3.99 mm/day to 35.69 °C, 51.31%, 17.92 mm/day, respectively.The variations of 222 Rn and RH had general trend with positive correlation 0.62, but there are some fluctuations in the data that make it difficult to determine if the trend is exactly the same.In contrast, 222 Rn behaves differently with AT and EV.This behavior can be explained by the fact that radon  www.nature.com/scientificreports/emissions from the ground and atmospheric dynamics control its concentration in the air.During the night and early morning when the atmosphere is stable, radon accumulates in the nearby air layers of the ground surface.After sunrise, as air temperature rises and moisture levels decrease, relative humidity in the air decreases, leading to increased vertical mixing of air, which moves towards the upper layers, resulting in a decrease in radon concentration despite its emission from the ground 16,17 .These findings are consistent with previous studies in the literature 15,[17][18][19] .The relationship between 222 Rn concentration and RH or EV can only be investigated when the temperature remains constant.Therefore, any observed relationships actually reflect the impact of temperature alone.It is important to note this fact when interpreting the data.

Variations of gaseous pollutants with outdoor 222 Rn
Rn and ground-level ozone (O 3 ) Ozone plays an important role in protecting from harmful ultraviolet radiation in the upper atmosphere (stratosphere), but it is considered a greenhouse gas and air pollutant at ground level.Therefore, the presence of high concentrations of ground level ozone is a source of concern due to its harmful effects on environmental systems 20 .The monthly average of ground level ozone (O 3 ) ranged from 21.18 µg/m 3 in January to 43.56 µg/m 3 in July, with yearly average of 31.68 ± 1.97 µg/m 3 .The concentrations of ground-level ozone were higher in the summer than in winter months (Fig. 6), possibly due to an increase in solar radiation intensity and air temperature, leading to an increase in ozone photochemistry and precursor emission rates 22 .Also, it is obvious from the Fig. 6, the concentrations of outdoor 222 Rn behaved in an opposite trend to ground level ozone, which can be explained by the fact that ozone concentration is highly dependent on atmospheric mixing.When the air boundary layers are mixed well (low radon concentration), the exchange between the lower and upper atmospheric layers is high, and the ozone concentration increases.While in the case of atmospheric stability (high radon concentration), the exchange between the upper and lower air layers decreases, and ozone removal mechanisms dominate, which leads to a decrease in its concentration 22 .

Rn and nitrogen oxides (NO x )
The major sources of nitrogen oxide in the air are fuel burning, emissions from industrial facilities, power plants, and vehicle exhaust.Nitrogen oxide can interact with oxygen and hydrocarbons through UV radiation forming ozone (O 3 ) and particulate matter (PM).The monthly average NO x concentration was in the range of 3.81-26.13µg/m 3 , with a yearly average value of 9.97 ± 2.27 µg/m 3 .These concentrations are below the allowable standard concentrations of 30 µg/m 3 suggested by EU, respectively 21,23 .Figure 7 shows the variations in 222 Rn and NO x concentrations, where it is clear that there is a similarity in those variations, as both 222 Rn and NO x are characterized by low levels in the summer.This can be attributed to the stable atmospheric conditions, characterized by weak mixing of air boundary layers, resulting in a notable rise in ground-level pollutant concentrations, including nitrogen oxides 13 .Additionally, these stable atmospheric conditions contribute to increased 222 Rn concentrations.Rn and particulate matter (PM 10 ) Anthropogenic activities, specifically traffic density, manufacturing processes, and road quality, are major contributors to PM, especially PM 10 .The monthly average of PM 10 concentration was in the range of 70.34-145.20µg/ m 3 , with a yearly average value of 109.95 ± 6.48 µg/m 3 .These annual values exceed the U.S. air quality standards and EU standards values (50 and 40 μg/m 3 , respectively) 21,23 .The higher PM 10 concentrations observed can be attributed to vehicle activities on unpaved roads, biomass, fossil fuel burning, and construction activities in the city of Qena 11 .The variations in the concentrations of 222 Rn and PM 10 are shown in Fig. 8.It is evident that the behavior of both is alike, with concentrations reaching their lowest values during the summer.This trend may be attributed to increased air circulation during the summer months, which is accompanied by a rise in temperature and a subsequent decrease in the concentrations of 222 Rn and PM 10 22 .

Pearson correlation coefficients
Pearson correlation coefficients were computed between 222 Rn and each of the meteorological parameters and gaseous pollutants using MS Excel software.The coefficients matrix is shown in Table 2 and     Among the air pollutants studied, only particulate matter had values higher than the acceptable levels, which reflects the potential health risks for the residents of the study area.The analysis revealed a direct correlation between outdoor 222 Rn concentration and RH, with both exhibiting high levels during winter and low levels during summer, supported by a positive correlation coefficient of 0.62.Conversely, a negative correlation was observed between 222 Rn and both AT and EV, with correlation coefficients of − 0.62 and − 0.73, respectively.Additionally, a direct relationship was observed between 222 Rn and both No x and PM 10 , with positive correlation coefficients of 0.31 and 0.41, respectively.In contrast, an inverse relationship was observed between 222 Rn and O 3 , with a negative correlation coefficient of − 0.72.These findings highlight the intricate connections between outdoor 222 Rn, meteorological parameters, and gaseous pollutants in the studied environment, emphasizing the need for continuous monitoring and mitigation efforts to combat air pollution and its associated health impacts.

Figure 5 .
Figure 5. Monthly average variations of 222 Rn with metrological parameters.

Figure 6 .
Figure 6.Monthly average variations of 222 Rn with O 3 .

Figure 7 .
Figure 7. Monthly average variations of 222 Rn with NO x .
the relations between them are shown in Figs. 9 and 10.The results obtained indicate significant correlations between 222 Rn and each of AT, RH, EV, and O 3 .Negative correlations were observed between 222 Rn and each of AT, EV, and O 3 , with correlation coefficients of − 0.62, − 0.73, and − 0.72, respectively.Positive correlations were observed between 222 Rn and each of RH, NO x , and PM 10 , with correlation coefficients of 0.62, 0.31, and 0.41, respectively.

Figure 8 .
Figure 8. Monthly average variations of 222 Rn with PM 10 .

Figure 9 .Figure 10 .
Figure 9.The relation between monthly average variations of 222 Rn and metrological parameters.

Table 1 .
Descriptive statistics of 222 Rn, metrological parameters, and gaseous pollutants in outdoor environment of Qena City, Egypt.

Table 2 .
Pearson correlation coefficients and P-value between222Rn, metrological parameters, and gaseous pollutants and atmospheric parameters.*The P-value ≤ 0.05.